Fluorescent lamps are used in a variety of environments to efficiently provide different levels of illumination. The conventional fluorescent lamp typically includes a glass tube having an electrode at each end of the tube. The tube is filled with mercury gas and another noble gas, such as argon. The inner surface of the glass tube is coated with phosphor. A drive voltage is supplied across the electrodes, causing the mercury atoms to emit ultraviolet photons. The emitted photons, in turn, excite the phosphorous, creating fluorescent illumination.
To power the conventional fluorescent lamp, the drive voltage is applied across the lamp electrodes, as stated above. The drive voltage is of sufficient magnitude to eject electrons from the electrodes into the tube. The ejected electrons collide with the mercury gas and excite the electrons of the mercury gas to higher energy levels. The collisions break down the mercury gas, causing electrons to flow across the length of the tube. This process of generating a current that flows through the tube is commonly referred to as striking an arc in the tube. While an arc is generated, the drive voltage causes electrons and the mercury atoms to collide with the argon gas. The electrons of the argon gas are first excited to high energy levels. Then the electrons drop down to low energy levels, thereby emitting infrared light.
The conventional fluorescent lamp can be used at various operating levels. The current provided to drive the fluorescent lamp is generally proportional to the illumination output of the fluorescent lamp. To operate the fluorescent lamp at a relatively high level of illumination, the current applied to the fluorescent lamp must be of correspondingly high magnitude. Lower levels of illumination are attained by reducing the magnitude of current provided to drive the fluorescent lamp. In this way, the fluorescent lamp is dimmed by appropriate control of the drive current.
At some threshold point, however, further reduction of the drive current will fail to further dim the fluorescent lamp in the "arc" mode. Rather, when the applied current falls below the threshold point, the arc and the fluorescent illumination it produces can no longer be generated, and a new mode of operation that will be described in more detail below is entered. There are reasons, some of which are discussed below, why it is undesirable to operate a fluorescent lamp in modes other than the arc mode. Accordingly, to dim the fluorescent lamp to low levels of illumination in the arc mode, the conventional fluorescent lamp drive circuit alternatively relies on pulsed applications of drive voltage to generate a discontinuous arc in the lamp. Under this technique, a relatively high drive voltage is briefly applied across the fluorescent lamp, striking a temporary arc in the tube. Then the drive voltage is removed for a predetermined time. Thereafter, a drive voltage is applied again. This technique repeats the application and removal of a relatively high voltage to give the appearance of a dimmed fluorescent lamp.
Many disadvantages stem from the use of conventional fluorescent lamp drivers. Perhaps the most significant disadvantages relate to the application of discontinuous, relatively high voltages to strike an arc. The application of discontinuous, relatively high drive voltages across the fluorescent lamp produces a voltage at the cathode that couples with the ionized gases in the tube. This voltage is commonly referred to as a cathode fall voltage. The cathode fall voltage accelerates the positively charged mercury atoms into the filaments of the cathode. If the cathode fall voltage is excessive, collisions between the mercury atoms with the filaments will cause particles of the filament to detach and accumulate near the ends of the inner surface of the tube in a process known as sputtering. Over time, sputtering can dramatically darken the ends of the tube. Such darkening of the tube significantly compromises the efficiency and durability of the conventional fluorescent lamp.
Another disadvantage of using pulsed applications of voltage to generate a discontinuous arc in the lamp is that the lower end of the illumination range is limited. More specifically, as the driving pulses get further and further apart, the human eye is able to detect a flickering in the lamp. This effect is undesirable for most applications.
The disadvantages of the conventional fluorescent lamp driver are readily apparent in applications requiring both high levels and low levels of illumination. Very often, conventional fluorescent lamps are used as backlighting for liquid crystal displays (LCDs). For example, the conventional fluorescent lamp could be implemented in an aircraft cockpit to illuminate a liquid crystal display. Bright sunshine penetrating the cockpit could make reading the liquid crystal display difficult. Therefore, the conventional fluorescent lamp must be capable of operating at a high level sufficient to adequately illuminate the liquid crystal display in such circumstances. Such high levels of illumination, however, require high drive currents which can mean excessive power levels if the driver is not designed properly.
The operation of the conventional fluorescent lamp at relatively low levels of illumination in connection with liquid crystal displays poses additional drawbacks. These drawbacks are especially problematic in military applications. Liquid crystal displays are used in numerous military environments, including, for example, aircraft instrument panels. The development of night vision technologies has required that conventional fluorescent lamps be operated at very low levels to illuminate a liquid crystal display while avoiding detection by night vision equipment. This requirement, however, is unsatisfied by the design of the conventional fluorescent lamp driver and its stimulation of argon and the subsequent emission of infrared light, which is readily detected by night vision equipment.
An exemplary prior art driver that has been able to obtain relatively high dimming ratios is disclosed in U.S. Pat. No. 5,420,481 to McCanney. The background section of the McCanney patent is particularly instructive, and selected sections are reproduced below. Much of the background section of McCanney, including FIG. 1, appears to have been taken from the textbook Electronics by Jacob Millman et al., (1941). FIG. 1 of the McCanney patent has been reproduced as FIG. 1 of the present application. It is notable that the curve shown in FIG. 1 may shift to the left or right along the current axis for different lamp technologies, although the shape of the curve should remain similar to how it is shown. In fact, for more recent lamp technologies the scale of the figure shown in the Millman textbook appears to be more accurate for the given current levels than FIG. 1 of the McCanney patent. In any event, as illustrated in FIG. 1, one of the reasons that fluorescent lamps are so difficult to drive is because the impedance of the lamp changes in a non-linear manner over a range of currents. Thus, the lamp often does not respond to a given input in a predictable manner. The McCanney patent begins its discussion of these complexities with the following description:
All fluorescent tubes are GAS GLOW DISCHARGE DEVICES. A study of the physics of glow or arc discharges in gaseous medium and of gas glowing discharge devices demonstrates that there are many complex and competing processes that produce and remove charges, which alter the ion population and the electric fields that direct them. The control of the current through a conductive, ionized gas is possible, but it is a complex process. The electrical conduction in gases and gas filled tubes encompasses a variety of effects and modes of conduction, ranging from the Townsend discharge at one extreme to the arc discharge at the other. The current ranges from a fraction of 1 microampere in Townsend discharge, to thousands of amperes in the arc discharge. A feature which distinguishes gaseous conduction from conduction in a solid is the active part which the medium plays in the process. Not only does the gas permit the drift of free charges from one electrode to the other, but the gas itself may be ionized to produce other charges which can interact with the electrodes to liberate additional charges. It will be shown below that the current voltage characteristic may be nonlinear and multivalued.
The McCanney patent goes on to discuss various other phenomena that affect the current voltage characteristics of a fluorescent lamp. In exploring the different phenomena that affect the current voltage characteristic of the lamp, McCanney discusses topics such as gaseous conduction, sources of free charge, net free charge concentration, motion of the charges, ion diffusion, and the mechanisms of conduction. For the four general regions A to D shown in FIG. 1, McCanney provides the following descriptions.